Update Eclair section of Build Custom Node Images

[rorp]

Description

This PR adds a working version of the custom Dockerfile for Eclair

Steps to Test

Follow the steps https://github.com/rorp/polar/blob/master/docs/custom-nodes.md, but use the updated Dockerfile.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (5677cf0) to head (b804c34).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1029   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          151       151           
  Lines         5513      5513           
  Branches      1110      1110           
=========================================
  Hits          5513      5513           

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